1. Technical Field
The present invention relates to photo lithographic lens systems and, more specifically, to a method and apparatus for obtaining quantitative measurements of the pupil plane illumination across the entire exposure field.
2. Background Art
Optical photolithography has been widely used in the semiconductor industry in connection with the formation of a wide range of structures in integrated circuit (IC) chips. Complex forms of pupil illumination patterns have become increasingly common for the purpose of improving resolution or depth of focus. The ability to measure and verify the correct distribution of illumination in the pupil plane has become increasingly critical.
Uniformity of the illumination at the wafer surface is required so that the same exposure of photoresist or other light sensitive films is consistently achieved across the entire exposure field. The degree of partial incoherency of the illumination, or more generally the distribution of pupil illumination, must also be constant across the entire exposure field. As tolerances of the printed lithographic patterns become increasingly tight, the requirement that the pupil illumination distribution not vary across the exposure field becomes increasingly important.
Various illumination systems for lithographic lenses have been developed, including those that create complex patterns of pupil illumination to enhance lithographic resolution and/or depth of focus. Illumination patterns, such as dipole, quadrupole, and annular shapes, have been developed to improve the resolution and depth of focus of the image formation. Some of these illumination patterns are particularly suited to enhancing the lithographic performance of specific mask patterns that are exposed on the stepper. When conventional partially coherent illumination is used, the center of the pupil is illuminated uniformly out to a prescribed fraction of the pupil size. In the case of both conventional partially coherent illumination and the more complex off-axis illumination patterns, the consistency of the illumination pattern at every position in the exposure field is critical.
Several techniques have been developed to adjust the uniformity and coherence of the illumination in lithographic exposure systems. For example, a spatial uniformity adjuster that can compensate for factors tending to deviate the illumination from uniformity is disclosed in U.S. Pat. No. 5,461,456 issued to Michaloski. An adjustable optical member that is refractive moves axially with respect to the lithography lens next to the pupil of the illuminator to help adjust the spatial uniformity of the illumination. Another method using a projection exposure apparatus that can change the coherency of the illumination is disclosed in U.S. Pat. No. 5,300,967 issued to Kamon. A reticle having openings for shaping light emitted from the light source and shielding patterns in order to reduce the area of the aperture, and thus affecting the coherency of the illumination, is used. In conjunction with these improved methods of adjusting the illumination, however, comes the need to measure and analyze the illumination patterns more carefully, to ensure that uniformity and consistency across the entire exposure field is achieved, and that the equipment is working properly and without defects.
The quality and uniformity of the illumination at the wafer plane can be analyzed and characterized by a variety of techniques, including wafer-plane power meters, analysis of photoresist or other light-sensitive films, etc. Measurement of the illumination pattern at the pupil plane(s) of the lithographic lens, however, is more difficult. Direct access to the entrance or exit pupil planes, or other planes optically conjugated to these planes, is usually not possible. For telecentric lithographic lenses, the pupils are virtual planes located at infinity. In the past, the pupil illumination has been measured by using a single, relatively large (one to a few millimeters), aperture in the plane of the photomask, this aperture flnction as a pinhole camera and projecting a geometrical image of the pupil illumination pattern. In order to capture this image with good resolution, the recording medium must be moved far enough away (several centimeters) from the wafer plane, which is the normal focal plane of the lithographic lens. Often the wafer chucking assembly must be removed in order to place the image-recording medium far enough from the focal plane to capture a good-quality image of the pupil illumination. Because of the large magnification of the image formed by this pin-hole camera technique, the intensity of the image is very low, and very sensitive recording mediums and/or long exposure times are needed. Also, because of the large magnification of the illumination pattern, typically only one pin hole image at a time can be formed within the exposure field of the lithographic lens. To test the pupil illumination pattern in several parts of the exposure field, the images and measurements would have to be made one at a time, since the images would overlap if two or more pinholes were present simultaneously in the exposure field.
Therefore, a need exists for a system that can efficiently obtain quantitative measurements of the illumination pattern at the pupil plane of the lithographic lens, so that analysis and verification of its regularity across the entire exposure field can be achieved.